Acoustic damping material



April 4, 1939. c WENTE 2,153,357

ACOUSTIC DAMPI NG MATERIAL Filed Nov. 13; 1936 FIG. 6 ,4 FIG. 7

/N VE N TOR y E.C.WENTE ATTORNEY Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application November 13, 1936, Serial No. 110,645

4 Claims.

This invention relates to acoustic treatment of rooms and the object of the invention is a non-porous wall covering material for improving the acoustic properties of rooms by absorption of sound energy through dissipation as the material is set in motion by the sound waves.

The common method of treating rooms to improve their acoustics is to cover the walls with a porous sound absorbing material but there are many disadvantages to this procedure. Since the pores must be kept open if the material is to be effective, they cannot be painted with ordinary paints, they are not easily cleaned, they do not harmonize with other decorative wall treatments, most of them are not fireproof, arid all of them are quite expensive. From a functional standpoint they are unsatisfactory in that they have excessive absorption at high frequencies and too little absorption at low frequencies.

It has also been proposed to cover the walls of auditoriums with wire mesh diaphragms spaced a short distance out from the wall and to cover the whole outer surface of the diaphragms witha smooth, continuous layer of ordinary plaster. These diaphragms are intended to resonate in a manner analogous to the action of sounding boards in musical instruments. The underlying theory is that good room acoustics requires resonant, rather than sound absorbing, walls. While this construction is ineffective acoustically it does have considerable appeal on the basis that it is supposed to provide the necessary acoustic correction without the disadvantages inherent in the use of porous materials.

In accordance with this invention the walls of the room to be treated are covered with large thin, lightweight diaphragms composed of metal, a thermoplastic material or any other suitable substance which is impervious to air flow. These diaphragms are spaced slightly away from the wall and are free to vibrate under the action of sound waves. In this method the structural advantages of the resonant wall described above are retained in that the surface is easily cleaned, may be fibrous material and may be painted or decorated in any desired manner without destroying its efiectiveness. One advantage of thermoplastic materials over metals is that they may be suitably colored in the process of manufacture so as to require no decorative treatment after installation.

The diaphragms being very thin and light in weight are readily set into vibration at large amplitudes but because of the low internal viscosity of some of the suitable materials, such as metals, a. simple diaphragm would dissipate very little energy and other means are provided to introduce the damping action required. The damping may take the form of a layer of viscous material such as the asphaltic compounds used in sound proofing applications, applied to the back of the diaphragm. The application of this material to the diaphragm increases its mass, however, without correspondingly increasing its stiffness and therefore reduces its response particularly at high frequencies so that in most cases a somewhat more complex structure utilizing some form of air damping is preferable. I

In the preferred structure according to the invention, a plurality of such diaphragms are mounted in close spaced relation and the damping is introduced by the viscous flow of air between them. While very thin spacers may sometimes be used to advantage they will not be required in most cases since the diaphragms will be in actual contact at only a few spots and the friction at these spots will add appreciably to the damping effect.

The amount of damping introduced may be controlled over a rather wide range by a proper choice of the number of diaphragms, the thickness of the spacers (when used) and the spacing between the rear diaphragm and the wall and in various other ways such as by providing a suitable acoustic impedance to air flow between the air spaces back of adjacent diaphragm units or by using book damping between the spaced diaphragms.

When the invention is used in locations where the fragile diaphragms are likely to be damaged, it will sometimes be advantageous to use a rear diaphragm of more rugged construction with perforations through which the air is forced when the fragile front diaphragm vibrates. The heavy diaphragm being mounted in close space relation to the vibratory diaphragm serves to support the latter when it is subjected to accidental impacts.

It is well understood in the art that for intelligibility of speech, in an auditorium a low reverberation time is required whereas a much higher reverberation time is desirable for the artistic reproduction of music.

According to a further feature of this invention, the reverberation time is made readily adjustable without affecting the appearance of the room by providing behind some or all of the diaphragms, electromagnetically operable controlling means for varying the diaphragm spacings or for moving suitable dampers in or out of contact with the diaphragm as required.

In the drawing:

Fig. 1 shows a portion of the corner of a room having walls covered with the diaphragms according to the invention;

Fig. 2 is a sectional view of a single diaphragm having a coating of viscous material;

Fig. 8 is a section of a double diaphragm unit with means for varying the damping effect;

Fig. 4 is a section of a multiple unit diaphragm;

Fig. 5 is a double diaphragm unit provided with damping means;

Fig. 6 shows an alternate form of double diaphragm unit; and

Fig. 7 shows a single diaphragm unit with an alternateform of variable damping control.

In general the diaphragms of the various units are rather large, preferably not less than twelve inches square and are formed of very thin nonporous material so that the whole diaphragm responds readily to sound waves over the frequency range of interest. For most suitable materials such as aluminum or other light metals, or the well-known thermoplastic molding materials, this requires a diaphragm of the order of from 1 to 10 mils in thickness and it will ordinarily be necessary to form such thin diaphragms with circular corrugations, as shown, or with other equivalent means for keeping them in shape.

The diaphragm ll of Fig. 2 comprises a central corrugation portion l2 and a flange 13 which spaces it about one inch or more as required from the face l4 of the wall on which it is mounted. In the forming process holes or grooves may be punched for the mounting nails 15 and the flanges may be cut away to permit a flow of air between adjacent diaphragm units. A layerof asphaltic or other viscous materials 16 of the proper thickness for the damping required, is applied to the back of the diaphragm to increase the dissipation of energy as the diaphragm vibrates under the action of sound waves. The amount of damping introduced may also be controlled to some extent at least by the nature of the communicating passages. A single large opening such as 11, Fig. 1, will permit a free flow of air but considerable dissipation may be introduced, when desired, by providing a suitable acoustic resistance in these air paths. This resistance may consist of a layer of cloth I! on the inner side of the flange or the flange itself may have a large number of very small slots ll of the type described in Patent 1,854,830 to I} B. Flanders, April 19, 1932.

While a single diaphragm unit is of relatively simple construction and may be satisfactory for some applications better results will ordinarily be obtained with a multiple diaphragm unit in which energy is dissipated by the viscous flow of the confined air. The structure of Fig. 3 comprises two similarly contoured diaphragms 2i and 22 which may be separated by spacers 23 a few mils in thickness. In most cases, however, the peripheral flanges 24, 25 may be in actual contact since the air confined between the diaphragms during assembly will keep them out of contact over most of the central vibratory portions. When the diaphragm 21 is vibrated by sound waves, the conflned air is displaced laterally between the diaphragms. Since the spacing is very small the impedance is resistive in nature and the dissipative effect is therefore substantially the same at all frequencies.

The unit of Fig. 3 may be provided with electromagnets 24, 21 disposed behind the rear diaamass? phragm and when this diaphragm is non-magnetic, small magnetic slugs 28, 29 may be secured therefore, will have desirable acoustic properties for speech or other sounds requiring low reverberation time. When a relatively live" condition is required for singing or instrumental music, for example, the magnets 28, 21 are energized and the diaphragm 22 is deflected inwardly, increasing the spacing between the diaphragms thereby materially reducing or substantially eliminating the damping and correspondingly increasing the reverberation time of the room. By connecting the magnets of groups of diaphragms in different portions of the room in suitable circuit arrangements, a system of this type provides a highly flexible, convenient and accurate control of the reverberation time without in any way ailecting the appearance of the room.

In the structure of Fig. 4 three (or more) similar diaphragms ll, 32, 33 are fitted together by pressing a groove 34 in the flange portions, or in any other suitable manner, secured together to form a unitary structure in which energy is dissi pated by a lateral flow of the confined air as in the structure of Fig. 3. The individual diaphragms may be composed of metal foil or sheets of thermoplastic material which are so thin and light in weight as to be capable of retaining their proper shape only when a considerable number of such diaphragms are used for each unit. With this construction, a high degree of damping over the whole frequency range of interest can be obtained.

Very effective air damping can also be obtained with a unit of the type disclosed in Fig. 5. The diaphragms 3B and 21 are assembled with somewhat greater spacing than in the construction shown in Fig. 3 and the enclosed space is loosely filled with a plurality of thin layers of flexible material such as paper or metal foil forming book damping members 28 of the type disclosed in Patent 1,583,067 granted to C. R. Moore on. May 4, 1926. The diaphragms are preferably very thin and the damping books" are preformed to the shape required to fill the space between the diaphragms. This unit is substantially the equivalent of one of the types shown in Fig. 4 and is relatively inexpensive.

In Fig. 6 the rear diaphragm 40 is spaced back of the front diaphragm 41 as in the construction shown in Fig. 3 but it is much thicker and stifler than the front diaphragm and is provided with a number of perforations 42. This construction is particularly suited for applications where the double thin diaphragm arrangement would not be rugged enough to withstand the accidental impacts which to which it is likely to be subjected The heavy diaphragm in close spaced relation to the thin front diaphragm serves as a stop mem her when the front diaphragm is excessively displaced by indentations and in this way serious damage to the front diaphragm is thereby avoided. Since the rear diaphragm is relatively stiff its amplitude of vibration will be small but the perforations permit a viscous flow of air which reduces the stiffness of the air space to the vahie required for the degree of damping desired.

In Fig. 7 behind the diaphragm 43 there is mounted on the pins 44 a damping bar 4! carrying a felt pad 46 which is normally pressed into light contact with the diaphragm by the springs 41. As in the construction of Fig. 3, when the magnets 48 are deenergized the diaphragm vibrations will be damped but when the magnets are energized and the bar is retracted, the damping is decreased or substantially eliminated and by a proper choice of the number and location of the units operated the reverberation time of the room may be increased to the extent desired in each case.

While the invention has been described with reference to particular embodiments, it will be understood that the structures illustrated may be modified in various Ways and used in any combinations desired and that the invention is intended to be limited only by the scope of the following claims.

What is claimed is:

1. A sound absorbing member for use in the acoustic treatment of rooms comprising at least two large diaphragms in close spaced relation whereby sound energy imparted to one of the diaphragms is dissipated by the viscous flow of air between the diaphragms, one of the diaphragms being very thin and impervious to air flow and another of the diaphragms having perforatlons for reducing the stiflness of the air confined between the diaphrasms.

2. A sound absorbing member for use in the acoustic treatment of rooms comprising two large diaphragms at least one of which is impervious to air flow and of such low mass and stifi'ness as to be readily vibrated by sound waves, and a plurality of layers of a thin flexible material impervious to air flow loosely filling the space between the diaphragms and forming air paths for the viscous lateral displacement of air between the layers.

3. A sound absorbing structure for the acoustic treatment of rooms comprising a plurality of adjacently disposed corrugated diaphragms of lightweight material not substantially more than 10 mils thick not less than 12 inches square, each of the diaphragms having flanges holding it in spaced relation to the wall of a room, said flanges having openings forming air paths between the spaces back of adjacent diaphragms.

4. A sound absorbing structure according to the preceding claim in which at least some of the air paths through the flanges have appreciable acoustic resistance.

EDWARD C. YWEN'I'E. 

